Currently, there are digital radio communication systems that allow, through their digital modulations, to provide an optimization of the features of the communications and the services provided to the users of said communications networks. An example of these technologies is the European standard TETRA, whose air interface for communications of voice plus data (V+D) is defined by the document ETSI EN 300 392-2 “Terrestrial Trunked Radio (TETRA) Voice plus Data (V+D)”. Said document contains the specifications for the physical layer, the data link layer and the network layer according to the ISO model.
In said standard, the services used for establishing traffic calls, for permitting both voice calls as well as data calls are defined, together with the state services of message data and of short data (Short Data Service SDS) of the TETRA standard, in addition to other services.
This technology is widely spread, accepted and established throughout the world to carry out professional communications of various security and emergency bodies, as well as for use in transport sectors and industrial sector companies.
Digital systems base their transmissions on digital modulations in which one or various bits form symbols and these, in turn, modulate a radiofrequency (RF) carrier signal in amplitude, phase, frequency or a combination of these. This is the case with TETRA technology which uses the modulation pi/4DQPSK.
In order to introduce systems of this type of technology, it is necessary that the regulatory authorities of different countries in which they are established, assign dedicated frequencies and frequencies of continuous usage for the operating of this type of network.
However, in certain countries, it is necessary to generate radio transmission identifiers, through analog modulations which require the emission of voice or Morse tones to be able to identify at a given time, the specific use of a frequency in the radio spectrum regulated in said country.
Currently, for example in USA and Canada, it is obligatory that all equipment that transmits a RF signal into the air, emit an identifier code, at given periods of time, every 15 or 30 minutes, according to each case, using voice or Morse code. This code is referred to in the regulations that regulate the use of the radio spectrum Call Sign or Station Identification, although there may also be other different names, for example, Continuous Wave Identification (CW ID).
The transmission of the identifier should be carried out using analog amplitude modulation (AM), frequency modulation (FM) or phase modulation (PM).
There are exceptions in which it is permitted that the equipment which use digital modulations can apply alternative methods to those mentioned. In the cases in which said exceptions are not permitted, the radio systems stop the transmission of the digital signal to send said identifier, in a way that produces a break in the service although not in the call itself, that is, if there is speech the break is not produced and the service is broken when the speaking is over.
The bodies that regulate the use of the radio spectrum are Federal Communications Commission (FCC) in the USA and Industry Canada (IC) in Canada. The regulations in which the identifier emission of the radio equipment is regulated and which are applied to the TETRA radio systems are the document Title 47 CFR Chapter I Part 90 in USA, specifically the points 90.425 and 90.647, and the document RIC-15 Issue 2 Radio Station Identification in Canada.
According to the previously mentioned, a digital radio communication system designed for maintaining continuous transmissions and thus being able to provide a continuous service over time, must interrupt the digital signal transmission at every certain time interval to generate by means of analog modulation, the identification of the base station coded in Morse, in those countries that require said signaling, such as USA and Canada. This currently means, for countries such as those mentioned in the example, that digital systems intended for critical missions, such as those for security and emergency services, are affected for a few seconds due to the imperative transmission of said Morse signaling every 15 to 30 minutes, which may cause the blocking of an emergency call for lack of availability of free traffic channels at a given time.
The present invention solves the interaction of the two transmissions, digital and analog, simultaneously maintaining a service continued by both technologies, avoiding the interruption of the digital transmissions and thus preserving the security of people in critical situations and who make use of said technologies.
As background of the state of the art, the U.S. Pat. No. 5,559,807 A, can also be cited, which shows a TDMA mobile communication system with a base station and a plurality of remote units.